Cover 2 baking sheets with parchment paper or silpats.
In a large heavy bottomed saucepan, combine the butter, sugar, syrup and salt. Cook over medium heat, whisking constantly until the butter is melted.
Bring to a boil, and cook, stirring constantly, until the mixture has reached 300-310° F (149-154° C) - DO NOT let it reach 320°F (160°C)
Remove toffee from heat and stir in vanilla.
Pour half the mixture onto each prepared baking sheet. (If you want, you can sprinkle some of the nuts on the baking sheet before pouring on the toffee).
Wait about a minute, until the toffee is set but still hot and sprinkle the chocolate chips over the top. Let chocolate soften for 5 minutes.
Spread the chocolate into a thin even layer (make sure chocolate is thick enough to hold the nuts).
Sprinkle with Maldon salt and almonds (or the pistachios).
Refrigerate until set. Break into pieces, and store in an airtight container. You will lose a few of the nuts when you break it up, but there should still be plenty left on top.